You may not. It may be that you are achieving good results, making steady improvement and enjoying the process. You may feel that you are running to your potential, or perhaps that’s not important to you so long as you’re improving. Kudos to you.
If you didn’t check all the boxes in the paragraph above, perhaps you’re a relatively new runner who has enjoyed the first blush of quick improvement, but now you’ve hit a plateau. Or you’re an experienced runner who has been doing similar training for years, and the results are beginning to disappoint. Or you’ve been doing well but you have always felt that there’s another level that you can reach.